Electrically-heated muffle



(No Model.)

W. MITCHELL.

- E'LEGTRIGALLY HEATED MUFFLE. I No. 491,438. Patented Feb. 7,1893.

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UNITED STATES PATENT rrrcn.

WVILLIS MITCHELL, OF MALDEN, ASSIGNOR TO THE AMERIOANELEOTRIC HEATING COMPANY, OF BOSTON, MASSACHUSETTS.

ELECTRlCALLY-HEATED MUFFLE.

SPECIFICATION forming part of Letters Patent No. 491,438, dated February 7, 1893.

Application filed July 5, 1892. Serial No. 438,950. (No model.)

To all whom/it may concern:

Be it known that I, WILLIS MITCHELL, a citi zen of the United States, residing at Malden, in the county of Middlesex and State of Massachusetts,have invented certain new and useful Improvements in Electric Metal-WVorking Devices; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention is an improvement on my application Serial No. 436,972, filed June 16, 1892; and it consists partly in the peculiar construction of the carbons therein employed and partly in a peculiarly constructed intervening muffle and the combination thereof with the said carbons.

In the accompanying drawings Figure 1 represents one of the carbons showing the peculiar distribution of material therein; Fig. 2 represents on a smaller scale a vertical section through the carbons the upper ends of the carbon-supports and the muffle; Fig. 3 represents a detail perspective view of the muiiie; and Fig. 4 represents a detail perspective view of the copper shell or backing for the carbon.

A and A designate the two carbon supparts which may have the general construction and arrangement shown in the application above mentioned.

B B designate the two carbons which are secured by screws 1) b in recesses of the heads of the said supports as in the said application. The rear or inner end of each of the said carbons is incased in acopper shell or backing O havingapproximately the form of a book cover. These shells or backings of copper are in communication with wires D D which extend through openings in the said supports and complete the circuit through the generator of electricity and the muiiie E. This muffle is a hollow casing, preferably rectangular as shown, having two parallel ledges e 6 formed externally on its sides, which receive the protruding ends of the carbons between them. Each of these carbons consists of a mixture of pulverized plumbago and pulverized fire clay or other material of high heat-resistance, that 7 part of the said carbon, marked 19 which is at the rear and within the copper shell having a greater proportion of the plumbago and the protruding part b of the said carbon having a less proportion of the plumbago. This latter part is that which holds the muffle and applies the heat. In consequence of this distribution of material the greatest production of heat will be at the sides of the muffle, for the purpose of acting on the metal or other substance inclosed therein to the best advantage. This mufiie is constructed of very refractory material and has also a higher electrical resistance than the carbon blocks or plates B B. When the circuit is closed the current of electricity generates an intense heat within it, which may be used for firing porcelain, fusing metals or any like purpose.

To insure better metallic connection 1 provide each shell or backing C with an intervening integral leaf of copper C and mold the mass of plumbago and fire clay into this shell and on both sides of this leaf, so that the latter is embedded within it. The proportions of fire clay and plumbago in the carbon block or plate thus formed may vary with the degree of heat desired and the work to be done. But the relative amount of more highly resisting material should be greatest at the heating end of each carbon.

Of course this method of construction or arrangement of particles may be adopted with other forms of carbons. Also other carbonaceous material or the equivalents thereof for such purposes may be substituted for plumbago. But I prefer everything as described and shown.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

l. A carbon consisting of a mixture of comminuted material of greater electrical resistance and comminuted material of less electrical resistance, the proportion of the former being greater at the heating end of the said carbon substantially for the purpose set forth.

2. A carbon composed of a mixture of plumbago and fire clay or other material of higher resistance to electricity, the proportion of fireclay being greater at the heat producing end on each of two opposite sides,in combination with carbon plates which fitbetween the said 15 ledges and support the said muffle and electric conductors making circuit through the said carbons and muffle substantially as set forth.

In testimony whereof I affix mysignature in 20 presence of two Witnesses.

WILLIS MITCHELL. Witnesses:

A. F. SARGENT, WILLIAM D. WRIGHT. 

